Add new UI features to the web viewer

[dellis23]

At the moment, it's somewhat difficult to move around. For me, the output is far off the screen, and I have to scroll down and to the size to see it. There also doesn't seem to be a way to zoom out, other than using the native browser zoom.

It's looking like this project could be very useful to us, so I'd be happy to work on this as I use it and contribute back the results if it's something you think would be valuable.

[dellis23]

One of the issues I've been having is an inability to find which bubbles are related to code in our repo, and which bubbles are related to outside libraries.

I've got a commit here (dellis23@7d67836) that adds a function to filter by filename. Javascript isn't my native tongue, so I'll have to hack a bit to get a clean widget on the page for actually allowing the user to do the input easily, rather than go through the console.

[bdarnell]

Yeah, the web interface could use a lot of work, and I'd welcome patches in this area - I don't know much javascript either. Smarter zooming and positioning is definitely a good idea.

Filtering is also needed. I had been thinking of filtering by stack sample instead of by node (as in the golang and google-perftools profiler), although doing that would require moving some logic (the computation of edges from stacks) that's currently on the server side up to javascript.

[bernii]

@bdarnell a did some polish to the web interface in the PR here #13